This invention relates to jet drop printers which print graphic information upon a sheet mounted on a rotary support member. In such recorders one or more print heads are mounted above the sheet and advance axially as the sheet is rotated repetitively thereunder. A typical recorder of this type is described in detail in Van Brimer et al U.S. Pat. No. 3,588,906.
The recorder of Van Brimer et al generates only a single jet for recording an image on the sheet. Such single jet operation is exceedingly slow and is impractical for many applications, such as office copying. Therefore it has been proposed that recording be carried out by means of apparatus capable of producing a plurality of jets. The difficulty with this is that the jets cannot be generated side-by-side with a sufficiently close spacing to achieve high resolution printing. Various techniques for circumventing the spacing problem are disclosed in Cahill et al 3,689,693, in Van Hook 4,009,332, in Paranjpe et al Ser. No. 789,417, and in Bruce -- IBM Technical Disclosure Bulletin Vol. 18, No. 12, May, 1976, page 3917.
In the Cahill arrangement there are provided a plurality of print heads, each of which produces a single jet. The heads are joined together for common stepping in the axial direction, so that each jet prints a solid band of graphic information. As the heads are stepped in the axial direction, the printed bands grow continually wider until they join. Printing of a full image is then complete, and axial stepping is terminated.
One of the problems with the Cahill arrangement has been that it is difficult to bring the printed bands together in exact registration. Such misregistration results in a visually apparent "seam" at boundaries between adjacent bands.
The arrangement of Paranjpe et al solves most of the problems of the prior art. As taught in Paranjpe et al a large number of spaced jets may be generated by a single head and printed in a spiral interlacing fashion. In order for this arrangement to work it is taught that the spacing between the jets should be such that it is equal to some number of image track widths which has no integer other than one as a factor in common with the number of nozzles. The speed of axial advance should be so related to the speed of drum rotation that during one drum rotation each nozzle is advanced an axial distance equal to a number of track widths corresponding to the total number of nozzles. While this enables printing at high speed and high resolution, it requires a print head which produces a fairly large number of jets.
Such heads tend to be somewhat expensive because of quality control problems associated with production of the necessary components. For instance, it is relatively easy to manufacture a high quality orifice plate or a high quality charge ring plate for production of ten or fifteen jets. As the number of jets increases cumulative tolerance errors also increase and the percentage yield of satisfactory parts decreases substantially. This is an important factor in applications such as office copiers, wherein cost must be minimized.
There is also a problem which arises due to the fact that the orifice plate must be moved from a position totally offset on one side of the paper to a position totally offset on the other. Thus the total travel distance for the print head is equal to the length of the drum plus twice the length of the orifice plate. For a high speed system the orifice plate is necessarily quite long, and thus a rather considerable amount of print head movement is required. This movement distance in turn creates a requirement for rather high print head movement speeds.
Van Hook and Bruce disclose jet drop printers which produce a series of circumferentially spaced jets. Bruce's jet producing nozzles are arranged in a single ring extending circumferentially around the printing drum, while Van Hook's nozzles are arranged in a plurality of rows. Either of these devices requires a data system capable of producing relatively long switching delays corresponding to the time of drum rotation from the first to the last circumferentially spaced printing position.